Molecular imaging may be defined as the three-dimensional mapping of molecular processes, such as gene expression, blood flow, physiological changes (pH, [O2] etc.), immune responses, and cell trafficking, in vivo. It can be used to detect and diagnose disease, select optimal treatments, and to monitor the effects of treatments to obtain an early readout of efficacy. A number of distinct technologies can in principle be used for molecular imaging, including positron emission tomography (PET), single photon emission tomography (SPET), optical (OI) and magnetic resonance imaging (MRI). Combinations of these modalities are emerging to provide improved clinical applications, e.g. PET/CT and SPET/CT (“multi-modal imaging”).
Radionuclide imaging with PET and SPET has the advantage of extremely high sensitivity and small amounts of administered contrast agents (e.g. picomolar in vivo), which do not perturb the in vivo molecular processes. Moreover, the targeting principles for radionuclide imaging can be applied also in targeted delivery of radionuclide therapy. Typically the isotope that is used as a radionuclide in molecular imaging is incorporated into a molecule to produce a radiotracer that is pharmaceutically acceptable to the subject. Many radiotracers have been developed with a range of properties. For example, fluorodeoxyglucose (18F) is a labelled glucose derivative that is frequently used in molecular imaging with PET.
WO 2009/021947 describes tripodal chelators for use as MRI contrast agents. Hydroxypyridinone chelating groups with a hydrophilic R group are described. The hydrophilic group is required to help solubilise the chelator. In addition, the chelator may be coupled to large molecules, such as a dendrimer, in order to increase the relaxivity of the MRI contrast agent.